This case-control study addresses the hypothesis that dietary cofactors are involved in the etiology of breast cancer. Previous epidemiologic studies have primarily examined fats in relation to breast cancer. While the experimental literature indicates that fats, (whether total fats, polyunsaturated fats, trans-isomer fatty acids) are positively associated with breast cancer risk, it also suggests that vitamins A, C and E, zinc, selenium, fiber and protease inhibitors are negatively associated with breast cancer risk, and that certain joint or interactive effects among these nutrients are important to take into account. This study will address these hypotheses in a human population. The subjects will be about 500 breast cancer cases and 750 controls obtained from a breast cancer screening program over a 6-year period. Information will be obtained on other risk factors for breast cancer. An extensive dietary questionnaire will be administered by telephone, and from this nutrient indexes will be calculated. Special methodological sub-studies will be performed to assess the reliability and validity of the dietary questionnaire. Analyses will use multivarate methods to control for confounding factors and to permit and examination of interactions in assessing the role of dietary factors in breast cancer.